The present invention relates to a projection exposure apparatus used in a photo-lithographic process to manufacture a semi-conductor device or a liquid crystal display device, and apparatus and method for detecting a mark position.
In a photo-lithographic process of a projection exposure apparatus for manufacturing a semi-conductor device or a liquid crystal display device, there has been used an exposure apparatus in which an image of a photo-mask or a reticle (referred to as xe2x80x9creticlexe2x80x9d hereinafter) on which a transfer pattern is formed is transferred onto a semiconductor wafer on which photosensitive material such as photoresist is coated or a photosensitive substrate such as a glass substrate (referred to as xe2x80x9cwaferxe2x80x9d hereinafter) by using a projection exposure method or a proximity exposure method. In such an exposure apparatus, since an image of a circuit pattern on the reticle must be overlapped with a circuit pattern already formed on the wafer with high accuracy, prior to exposure, positional alignment between the reticle and the wafer (referred to as xe2x80x9calignmentxe2x80x9d hereinafter) should be performed. The exposure apparatus performs the alignment by correctly detecting a position of each positioning mark (referred to as xe2x80x9calignment markxe2x80x9d hereinafter) formed on the wafer as well as a position of the circuit pattern in the previous process.
There are the following alignment systems. In an LSA (laser-step-alignment) system, as shown in U.S. Pat. No. 5,151,750, laser light flux is illuminated on a grid-shaped alignment mark and the position of the alignment mark is detected on the basis of change in intensity of reflected light. In an LIA (laser interference alignment) system, as shown in U.S. Pat. No. 5,151,750, coherent light fluxes are incident on a grid-shaped alignment mark from two symmetrical order directions (for example, +first-order diffraction light direction and xe2x88x92first-order diffraction light direction) and the position and positional deviation (in a pitch direction) of the grid mark by interfering two diffraction light components generated from the grid mark along the same direction. Further, in an FIA (field-image-alignment) system, as shown in U.S. Pat. No. 5,493,403, in a condition that a wafer stage is stopped, white light is illuminated onto an alignment mark on the wafer, and an image of the alignment mark so obtained is detected by an imaging element, the position of the alignment mark is detected by effecting image treatment process.
There are the following alignment optical systems used with such alignment systems. In a TTL (through-the lens) system, a projection optical system is used as a detection optical system and the alignment mark on the wafer is detected by the projection optical system. In a TTR (through-the-reticle) system, a reticle alignment mark provided on the reticle and the alignment mark on the wafer are simultaneously observed (detected) through a projection optical system. In an off-axis system, there is provided an exclusive optical system having an optical axis spaced apart from an optical axis of a projection optical system by a predetermined distance, the alignment mark on the wafer is observed (detected) while illuminating white light from an exclusive light source on the alignment mark.
In the past, since a wafer stage on which the wafer to be moved is rested has a predetermined positional relation to a wafer stage guide through roller bearings and is position-controlled by a rotating motor and an associated ball screw, so long as the motor is stopped, the wafer stage could easily be maintained in the stopped condition. Recently, as a so-called scan type projection exposure apparatus in which a reticle stage on which the reticle is rested and the wafer stage are scanned to perform exposure in a synchronous manner has been developed, since high speed driving and minute control of the wafer stage are required, a so-called air stage of complete non-contact type in which the stage is floated by air pressure and is driven by a linear motor of non-contact type has been used. Although the air stage is superior to the conventional stages in various points such as a moving speed, dynamic control and the like, since it is a complete non-contact type, when observed microscopically, it is difficult to maintain the stage in the stopped condition.
In the projection exposure apparatus in which the air stage of complete non-contact type which is always moved microscopically is used as the wafer stage, for example, if the alignment is effected by using the off-axis alignment optical system in the FIA system, the following problems will occur.
In the image treatment alignment using the conventional white light source, since a predetermined integrated exposure amount must be provided to an imaging element used for picking-up the image, emission of the white light illuminated onto the alignment mark on the wafer must be continued for a predetermined time period. Accordingly, during the emission of the white light, the wafer stage on which the wafer is rested must be kept stationary. If the position of the wafer stage is shifted during the emission of the light for giving the integrated exposure amount required for the imaging element, accuracy of detection of the position of the alignment mark will be worsened.
However, in the air stage of complete non-contact type as mentioned above, since it is difficult to keep the stage stationary, the position of the air stage may be minutely changed while the imaging element of the alignment optical system is picking-up the image of the alignment mark, with the result that the accuracy of detection of the position of the alignment mark will be worsened, thereby making the high accurate alignment impossible.
Although it is considered that the air stage is mechanically stopped during the detection of the alignment mark by using any fixing mechanism, this attempt is not preferable because mechanical stress is applied to the air stage whenever the air stage is forcibly stopped and an excessive time period is required for driving the fixing mechanism to fix the air stage, resulting in reduction of through-put. If a white light source of continuous emission type having illuminance capable of obtaining the required integrated exposure amount of the imaging element for so short time period so that the shift of the position of the air stage is negligible, a heat amount generating from the white light source would be considerably increased, with the result that heat shield of the exposure apparatus is made difficult.
As another related technique, in a mark position detecting apparatus of image focusing type used in such an exposure apparatus, both an image of pinch marks disposed side by side at a predetermined interval (i.e., index pattern) formed on the reticle and an image of an alignment mark (for example, line-and-space mark) on the photosensitive substrate such as a wafer or a glass substrate are focused onto an imaging element through a projection optical system a focusing optical system, whereby a position of the photosensitive substrate is detected on the basis of image data from the imaging element. Further, it is also known to provide a mark position detecting apparatus in which an index plate having a substantially conjugate relation to a wafer is provided within an alignment optical system via an objective optical system different from a projection optical system, and both an image of an index pattern on the index plate and an image of the wafer alignment mark are focused onto an imaging element through a focusing optical system, and a position of the photosensitive substrate is detected on the basis of image data from the imaging element. In such position detecting apparatuses, the index pattern and the alignment mark are imaged (or photo-taken) by several times to obtain a plurality of images, and these plural images are averaged to improve measuring accuracy.
However, When the image data from the imaging element is obtained, in order to improve SN of the image, image accumulation time of the order of 10 msec is required. During this time period, if the wafer stage is minutely shifted, the mark image would become obscure. Thus, in the related position detecting apparatuses, before the image data is obtained, the wafer stage is positioned at a predetermined position accurately within an error range of several nmxcx9c10 nm, and, after the wafer stage is completely stopped, the image data is obtained. Therefore, the time period for positioning the stage prior to the alignment is increased, thereby reducing the through-put. Further, when the averaging treatment is effected by using the several images, if the image data are merely added to each other, the composite mark image after addition would become obscure in dependence upon the shift (or positional deviation) of the wafer stage during the acquirement of the image data.
A first object of the present invention is to provide a projection exposure apparatus in which alignment can be effected with high accuracy by using a light source emitting white flashing light as detection light of an alignment system even when a position of a wafer stage is shifted microscopically.
A second object of the present invention is to provide mark position detecting apparatus and method, in which an alignment mark is imaged while a substrate stage is being shifted, and averaging treatment is effected to prevent obscure (or blurred) image.
To achieve the above first object, according to the present invention, there is provided a projection exposure apparatus comprising a projection optical system PL which projects an image of a pattern on a reticle illuminated by exposure light onto a photosensitive substrate, a substrate stage WST which shifts the photosensitive substrate W in a plane perpendicular to an optical axis of the projection optical system, a stage position measuring system WST1, WST2 which measures a shift amount of the substrate stage, and an alignment system 1 which illuminates detection light onto an alignment mark AM provided on the photosensitive substrate and receives reflected light reflected from the alignment mark to detect a position of the alignment mark, and wherein the alignment system has a light source 2 emitting white flashing light.
Preferably, a timing for emitting the flashing light is in synchronous with a measuring timing of the stage position measuring system.
Preferably, the flashing light is emitted by several times, and respective positions of the substrate stage upon emission of the respective flashing lights are measured by the stage position measuring system, and a plurality of detected images of the alignment mark are averaged.
According to the present invention, a required integrated exposure amount can be given to an imaging element of an image treating alignment system within a light emitting time period of the flashing light from the light source, and, since the light emitting time period of the flashing light is very short, a shift amount of the wafer stage within the light emitting time is negligible. That is to say, by the control at an alignment signal treating system, a light emitting timing of the flashing light of the flash lamp (light source) is in synchronous. with a measuring timing of a wafer stage interferometer so that the taking-in of the alignment mark and the position measurement of the wafer stage are effected simultaneously, with the result that, even if the position of the wafer stage is shifted microscopically, high accurate alignment can easily be effected.
To achieve the above second object, according to the present invention, there is provided a mark position detecting apparatus comprising a position detecting illumination system 205 which illuminates pulse light onto an alignment mark WM1 on a substrate W, an imaging element S1 which images the alignment mark WM1 in synchronous with emission of the pulse light, a position detecting system 107 which reads a position of a substrate stage 105 upon the imaging by the imaging element, a position memory system 300 which stores the position of the stage 105 read by the position detecting system 107, and an image memory system 300 which stores image data imaged by the imaging element S1.
Preferably, there is provided an image composing system which forms a composite image on the basis of the image data stored in the image memory system 300 and the position data corresponding to the plurality of image data stored in the position memory system 300.
Further, preferably, when the signals of respective pixels of the imaging element S1 are added to each other with respect to the plurality of image data, the image composing system corrects the signals of respective pixels by shift amounts of the stage 105 detected by the position detecting system 107 at respective imaging points and then are added.
Further, preferably, the mark WM1 is imaged by plural times while shifting the stage 105.
Further, to achieve the above second object, according to the present invention, there is provided a mark position detecting method comprising an illuminating step for illuminating pulse light onto an alignment mark on a substrate W, an imaging step for imaging the alignment mark in synchronous with emission of the pulse light, a position detecting step for reading a position of a substrate stage at the imaging point, a position storing step for storing the position of the stage read by the position detecting step, and an image storing step for storing image data so imaged.
Preferably, there is added an image comprising a step for forming a composite image on the basis of the image data stored in the image storing step and the position data corresponding to the plurality of image data stored in the position storing step.
Further, preferably, when the signals of respective pixels of the imaging element S1 are added to each other with respect to the plurality of image data, the signals of respective pixels are corrected by shift amounts of the stage 105 detected at respective imaging points and then are added.
Further, preferably, the mark is imaged by plural times in synchronous with the pulse light while shifting the stage 105.
According to the above-mentioned apparatus and method, since the position detecting light source of pulse generating type is used, it can be assumed that the stage is substantially stopped within the pulse generating time period, and, thus, even when the mark is imaged while the stage is not completely stopped, the mark image does not become blur. Particularly, when the images are added (or composed) on the basis of the plurality of pulses, since the positional deviation of the stage at each imaging point is corrected, the composite mark image (i. e., added mark images) does not become blur. In addition, when the position is detected, since the accurate positioning is not required, after rough positioning is effected, even if the wafer stage is minutely shifted, the image data can be obtained, with the result that the total alignment time can be reduced, thereby improving the through-put. That is to say, according to the present invention, even when the mark is imaged by plural times to improve the alignment accuracy, desired through-put and desired accuracy can be achieved.